1. Field of the Invention
The present invention relates to a plunger pump that draws and discharges liquid as the plungers reciprocate. More particularly, the invention relates to a dispenser of plunger-pump type, in which three plungers revolve and reciprocate and a planer valve switches the flow of the liquid, thereby to move the liquid in proportion to the revolution of the plungers.
2. Description of the Related Art
A dispenser of plunger-pump type is known, which can discharge in a very small amount with high precision and can apply an adhesive in methods of manufacturing semiconductor devices. (Such dispensers are disclosed in, for example, Japanese Patent Application Laid-Open Publication No. 6-129345.)
This dispenser comprises a body, a valve block and a pump block. The valve block has a sliding surface, at which a communication hole opens. The communication hole communicates with an inlet port and an outlet port. The body supports the pump block, allowing the pump block to rotate. The pump block has a sliding surface at one end. The pump block abuts, at the sliding surface, on the sliding surface of the valve block. The pump block has three plunger holes. These holes can communicate with the communication hole of the valve block and extend in the axial direction of the pump block. While biased onto the valve block, the pump block can rotate to make the plunger holes sequentially communicate with the communication hole. The plungers held in the plunger holes are driven in their axial direction to draw and discharge liquid repeatedly. The dispenser can, therefore, discharge the liquid in an extremely small amount and at a constant rate, without causing a pulsating motion of the liquid.
The dispenser has an end cam, which makes the plungers reciprocate while revolving, and rollers that abut on the end cam. The rollers are used as cam followers. They are coupled to the rods that drive the plunges.
Each roller rotates, while revolving. Inevitably, it rotates at different speeds at the inner and outer ends of the radius of revolution if the plane in which each roller contacts the end cam is flat. Consequently, the roller undergoes lateral sliding. To minimize the wear of the rollers, the end cam is made of oil-impregnated resin or the like. If the end cam made of oil-impregnated resin is used, its surface will deform, reducing the operating precision of the dispenser, i.e., the rate at which the dispenser discharges the liquid.
To prevent the lateral sliding, the roller may have a conical surface. If the roller has a conical surface, however, a thrust will be generated. Consequently, the conventional dispenser must be complex in structure in order to eliminate such a thrust.
Since the axle, on which the roller is mounted to rotate, protrudes from the driving rod, the radius of revolution of the roller cannot be as short as desired. The miniaturization of the dispenser is inevitably limited.